In the field of digital radio communication, packet communication represented by Internet communication, speech storage and so on, speech signal encoding and decoding techniques are essential for effective use of channel capacity and storage media for radio waves. In particular, a CELP speech encoding and decoding technique is a mainstream technique (for example, see non-patent document 1).
A CELP speech encoding apparatus encodes input speech based on speech models stored in advance. To be more specific, the CELP speech encoding apparatus divides a digital speech signal into frames of regular time intervals, for example, frames of approximately 10 to 20 ms, performs a linear prediction analysis of a speech signal on a per frame basis to find the linear prediction coefficients (“LPC's”) and linear prediction residual vector, and encodes the linear prediction coefficients and linear prediction residual vector individually. A CELP speech encoding or decoding apparatus encodes or decodes a linear prediction residual vector using an adaptive excitation codebook storing excitation signals generated in the past and a fixed codebook storing a specific number of fixed-shape vectors (i.e. fixed code vectors). Here, while the adaptive excitation codebook is used to represent the periodic components of a linear prediction residual vector, the fixed codebook is used to represent the non-periodic components of the linear prediction residual vector that cannot be represented by the adaptive excitation codebook.
Further, encoding or decoding processing of a linear prediction residual vector is generally performed in units of subframes dividing a frame into shorter time units (approximately 5 ms to 10 ms). In ITU-T Recommendation G.729 disclosed in Non-Patent Document 2, an adaptive excitation is vector-quantized by dividing a frame into two subframes and by searching for the pitch periods of these subframes using an adaptive excitation codebook. Such a method of adaptive excitation vector quantization in subframe units makes it possible to reduce the amount of calculations compared to the method of adaptive excitation vector quantization in frame units.    Non-Patent Document 1: M. R. Schroeder, B. S. Atal “IEEE proc. ICASSP” 1985, “Code Excited Linear Prediction: High Quality Speech at Low Bit Rate┘, pages 937-940    Non-Patent Document 2: “ITU-T Recommendation G.729,” ITU-T, 1996/3, pages 17-19